The TSA has been secretly monitoring US airline passengers using air marshals

The TSA has been secretly tracking airline passengers through a recently revealed domestic surveillance program called Quiet Skies, the Boston Globe reported.

Quiet Skies uses federal air marshals to monitor individuals that are not under investigation by another intelligence agency and not in the terrorist database.

According to the TSA, Quiet Skies has been around since 2010.

The Transportation Security Administration has been secretly monitoring US airline passengers for years, the Boston Globe reported.

According to the publication, federal air marshals have been covertly surveilling individuals that "are not under investigation by any agency and are not in the Terrorist Screening Data Base."

The surveillance is part of a recently revealed domestic surveillance program, called Quiet Skies, that aims to head off threats to commercial aviation by "unknown or partially known" terrorists. However, the Boston Globe's reporting indicates that the program has called for the monitoring of 40 to 50 private citizens per day, many of whom seemingly pose no security threat, such as a businesswoman, a flight attendant, and even a federal law enforcement official.

At the heart of Quiet Skies is the observation of individuals for certain behaviors, such as whether they were abnormally aware of their surroundings or were excessively nervous. In addition, their appearance would be scrutinized for changes in comparison to their photo identification. Other passenger behavior would also be noted, such as whether they checked bags or slept on the flight.

A TSA spokesperson confirmed to Business Insider that Quiet Skies has been in existence since 2010.

In a statement, the TSA equated the Quiet Skies to "putting a police officer on a beat" and offered assurances that the intelligence-gathering activity includes "robust oversight."

In addition, the TSA also pushed back against the publication's reporting.

"Contrary to the article 'Welcome to the Quiet Skies' published by The Boston Globe, the program doesn't take into account race and religion, and it is not intended to surveil ordinary Americans," the agency said.

Here is the TSA's statement in its entirety:

"The purpose of this program is to ensure passengers and flight crew are protected during air travel. Contrary to the article 'Welcome to the Quiet Skies' published by The Boston Globe, the program doesn't take into account race and religion, and it is not intended to surveil ordinary Americans. In the world of law enforcement, this program's core design is no different than putting a police officer on a beat where intelligence and other information presents the need for watch and deterrence. The program analyzes information on a passenger's travel patterns, and through a system of checks and balances, to include robust oversight, effectively adds an additional line of defense to aviation security. With routine reviews and active management via legal, privacy and civil rights and liberties offices, the program is a practical method of keeping another act of terrorism from occurring at 30,000 feet."